Method of fabricating a top-gate type thin film transistor with dangling bonds of silicon partly combined with hydrogen

ABSTRACT

In a top-gate type thin film transistor including a polycrystalline silicon pattern having a channel region, a source region and a drain region on a substrate, a gate electrode via a gate insulating layer on the polycrystalline silicon layer, an insulating layer thereon, and metal electrodes coupled to the source region and the drain region, dangling bonds of silicon of the channel region at an interface with the gate insulating layer and dangling bonds of silicon of a part of the drain region are combined with hydrogen.

This is a continuation of application Ser. No. 08/636,755 filed Apr. 19,1996, now U.S. Pat. No. 5,888,856, which is a divisional of applicationSer. No. 08/561,334, filed Nov. 21, 1995, now U.S. Pat. No. 5,693,961,the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a top-gate type polycrystalline siliconthin film transistor (TFT).

2. Description of the Related Art

Polycrystalline silicon TFT's are used in integrated circuits,particularly, load elements of a static random access memory (SRAM) andliquid crystal devices (LCD's).

In a prior art method for manufacturing a top-gate type TFT, apolycrystalline silicon layer, a gate insulating layer, a gate electrodelayer, a non-doped insulating layer, and a metal connection layer areformed on a substrate, and then, a hydrogen passivation usinghydrogenation by plasma discharge is carried out, to thereby reduce trapstate densities of the polycrystalline silicon layer and improve theperformance of the TFT, That is, a hydrogen passivation time is so longthat saturated trap reduction characteristics and saturated thresholdvoltage characteristics can be obtained (see: I-WEI WU et al. "Effect ofTrap-State Density Reduction by Plasma Hydrogeneration inLow-Temperature Polysilicon TFT", IEEE ELECTRON DEVICE LETTERS, VOL. 10,No. 3, pp. 123-125, March 1989 and "Performance of Polysilicon TFTDigital Circuits Fabricated with Various Processing Techniques andDevice Architechtures", SID 90 Digest, pp. 307-310, 1990). This will beexplained later in detail.

In the above-described prior art method, when a gate length of the TFTis too small, for example, less than 10 μm, a parasitic bipolarphenomenon may occur, so that the electric property is fluctuated. Forexample, the breakdown voltage of the TFT is reduced, and the thresholdvoltage of the TFT is fluctuated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a top-gate type TFTcapable of suppressing the reduction of the breakdown voltage and thefluctuation of the threshold voltage.

Another object is to provide a method for manufacturing such a top-gatetype TFT.

According to the present invention, in a top-gate type thin filmtransistor including a polycrystalline silicon pattern having a channelregion, a source region and a drain region on a substrate, a gateelectrode via a gate insulating layer on the polycrystalline siliconlayer, an insulating layer thereon, and metal electrodes coupled to thesource region and the drain region, dangling bonds of silicon of thechannel region at an interface with the gate insulating layer anddangling bonds of silicon of a part of the drain region are combinedwith hydrogen.

Thus, in the channel region, since only dangling bonds of silicon at theinterface of the channel region and the gate insulating layer arecombined with hydrogen, no parasitic bipolar phenomenon occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription as set forth below, in comparison with the prior art, withreference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional diagram illustrating a prior art top-gatetype TFT;

FIG. 2 is a graph showing I_(DS) -V_(G) characteristics of the TFT ofFIG. 1;

FIG. 3 is a graph showing trap state reduction characteristics andthreshold voltage characteristics of the TFT of FIG. 1 dependent upon ahydrogen passivation time period;

FIGS. 4A through 4J are cross-sectional views illustrating a firstembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention;

FIG. 5 is a plan view of the TFT of FIG. 4J;

FIG. 6 is a graph showing hydrogen passivation time in relation to drainvoltage characteristics of the TFT of FIG. 4J and 5;

FIG. 7 is a graph showing hydrogen passivation time in relation tothreshold voltage characteristics of the TFT of FIG. 4J and 5;

FIG. 8 is a graph showing hydrogen peak intensity characteristics forestimating the amount of hydrogen trapped in the polycrystalline siliconpattern of FIGS. 4J and 5;

FIGS. 9A and 9B are cross-sectional views illustrating devices used forobtaining the characteristics of FIG. 8;

FIG. 10 is a graph showing degassed heavy hydrogen characteristics ofthe device of FIG. 4B;

FIGS. 11A through 11B are cross-sectional views illustrating a secondembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention;

FIG. 12 is a plan view of the TFT of FIG. 11B;

FIG. 13 is a graph showing current to voltage characteristics of the TFTof FIG. 11B and 12;

FIGS. 14A through 14E are cross-sectional views illustrating a thirdembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention;

FIGS. 15 is a plan view of the TFT of FIG. 14E; and

FIGS. 16A through 16F are cross-sectional views illustrating a fourthembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the preferred embodiments, a prior art methodfor manufacturing a TFT will now be explained with reference to FIG. 1(see: the above-mentioned documents by I-WEI WU et al.).

In FIG. 1, a prior art top-gate type TFT is illustrated. That is, a 100nm thick amorphous silicon is deposited on a fused quartz substrate 101by a low pressure chemical vapor deposition (LPCVD) process, and aheating operation is performed thereupon at a temperature of 600° C. forfour hours in a nitrogen atmosphere, to thereby from a polycrystallinesilicon layer 102. The polycrystalline silicon layer 102 is patternedinto an island. Then, a 100 nm thick gate silicon oxide layer 103 isformed, and also, a 350 nm thick polycrystalline silicon gate electrodelayer 104 is formed. After the gate electrode layer 104 is patterned,2×10¹⁵ phosphorous ions/cm² are doped in self-alignment with thepatterned gate electrode layer 104, to form N⁺ -type source and drainregions in the polycrystalline silicon layer 102. Then, a 700 nm thicknon-doped low temperature silicon oxide (LTO) layer 105 is deposited bya LPCVD process, and a heating operation at 600° C. is carried out toanneal the doped impurity ions within the source and drain regions ofthe polycrystalline silicon layer 102. Thereafter, contact holes areperforated within the LTO layer 105 and the gate silicon oxide layer103, and a 1 μm thick AlSiCu layer 106 is deposited. Then, the device ofFIG. 1 is sintered at a temperature of 450° C. for 30 minutes within aH₂ --N₂ forming gas. Finally, a hydrogen passivation is carried out for16 hours in a parallel-plate plasma reactor at a substrate temperatureof 350° C. with an H₂ and Ar gas mixture at a power density of 0.21W/cm² and a frequency of 30 kHz. As a result, dangling bonds of siliconof the polycrystalline silicon layer 102 are completely combined withhydrogen. Thus, the TFT is completed.

In the TFT of FIG. 1, hydrogen atoms can diffuse in the 700 nm thicknon-doped LTO layer 105 to reach the active channel region of thepolycrystalline silicon layer 102. Therefore, as shown in FIG. 2, when agate length L is 50 μm, a gate width W is 20 μm, and a drain voltageV_(DS) is 5V, the drain current (I_(DS))-to-gate voltage (V_(G))characteristics are improved. That is, a leakage current is decreasedsome hundred times (≈10⁻²), an ON current is increased some thousandtimes (≈10³) at the gate voltage V_(G) =20V and some hundreds ofthousands of times (≈10⁵) at the gate voltage V_(G) =5V. Also asubthreshold voltage characteristic is improved.

Also, as shown in FIG. 3, which shows the trap state reduction andthreshold voltage characteristics of the polycrystalline silicon layer102, when the hydrogen passivation time is longer than 16 hours,saturated trap state reduction characteristics and saturated thresholdvoltage characteristics are obtained.

The TFT of FIG. 1 can be applied to a case where the gate length L isrelatively large, such as, L=50 μm. However, when the gate length Lbecomes small, for example, when L=10 μm, the electric property of theTFT is fluctuated.

That is, the P⁻ -type channel region of the polycrystalline siliconlayer 102 is in a floating state, and accordingly, the voltage V_(BODY)at the channel region is dependent upon the drain voltage V_(DS). Also,when the voltage V_(BODY) at the channel region reaches 0.6V, aparasitic bipolar phenomenon may occur to thereby fluctuate theelectrical property of the TFT. Therefore, the drain voltage V_(DS) isrequired to be sufficiently high, by which the voltage V_(BODY) at thechannel region reaches 0.6V.

However, the insulating layer 105 is made of a double structure ofnon-doped silicon oxide and flattened boron-including phospho-silicatedglass (BPSG) and the gate length L is smaller than 10 μm. Therefore,when a hydrogen passivation is carried out for 16 hours, the drainvoltage is smaller than 6V when the voltage V_(BODY) at the channelregion reaches 0.6V. As a result, the breakdown voltage is reduced, andthe threshold voltage is fluctuated. In view of the requirement for a±20 percent fluctuation of the power supply voltage, the prior art TFTof FIG. 1 cannot be applied to 5V system devices.

FIGS. 4A through 4J are cross-sectional views illustrating a firstembodiment of the method for manufacturing a top-gate TFT according tothe present invention.

First, referring to FIG. 4A, a non-doped silicon oxide layer 2 isdeposited on a fused quartz substrate 1 by a CVD process. In this case,other insulating layers made of silicon nitride or a stackedconfiguration of non-doped silicon oxide and BPSG can be used instead ofthe non-doped oxide layer 2. Also, if a monocrystalline siliconsubstrate is used instead of the fused quartz substrate 1, the non-dopedsilicon oxide layer 2 is grown by thermally oxidizing themonocrystalline silicon substrate.

Then, an about 80 nm thick amorphous silicon layer is deposited by anLPCVD process at a substrate temperature of about 500° C. with a sourcegas of Si₂ H₆. Then, the amorphous silicon is heated for about 12 hoursat a temperature of about 600° C. in a nitrogen atmosphere to change theamorphous silicon into a polycrystalline silicon layer 3. In this case,before or after the conversion of the amorphous silicon into thepolycrystalline silicon layer 3, about 2×10¹⁷ boron ions per cm² areimplanted thereinto, so that the polycrystalline silicon layer 3 is of aP⁻ -type.

Next, referring to FIG. 4B, the polycrystalline silicon layer 3 ispatterned into an island-shaped polycrystalline silicon pattern 3a.

Next, referring to FIG. 4C, an about 100 nm thick non-doped gate siliconoxide layer 4 is formed by an LPCVD process. Then, an about 200 nm thickpolycrystalline silicon layer 5 is formed by an LPCVD process.

Next, referring to FIG. 4D, the polycrystalline silicon layer 5 ispatterned into a gate electrode layer 5a.

Note that the gate electrode layer 5a can be made of polycide, silicideor metal.

Next, referring to FIG. 4E, about 1×10¹⁵ phosphorous ions/cm² areimplanted at an energy of about 50 keV into the polycrystalline siliconpattern 3a in self-alignment with the gate electrode layer 5a. As aresult, an N⁺ -type source 31 and an N³⁰ -type drain region 32 areformed in self-alignment with the gate electrode layer within thepolycrystalline silicon pattern 3a. Also, simultaneously, a P⁻ -typechannel region 33 is formed within the polycrystalline silicon pattern3a between the N⁺ -type source region 31 and the N⁺ -type drain region32.

Next, referring to FIG. 4F, an about 50 nm thick non-doped silicon oxidelayer is formed by an LPCVD process, and then, an about 350 nm thickBPSG layer is formed by an atmospheric pressure CVD (APCVD) process.Thus, an insulating layer 6 made of silicon oxide and BPSG is formed.Then, a heating operation is carried out at about 900° C. for about 30minutes to anneal the source region 31 and the drain region 32 andflatten the insulating layer 6. In this case, the non-doped siliconoxide of the insulating layer 6 protects the active region of the TFT,i.e., the polycrystalline silicon pattern 3a. Also, since the BPSG ofthe insulating layer 6 includes impurities such as boron andphosphorous, the non-doped silicon oxide of the insulating layer 6prevents such impurities from being diffused into the polycrystallinesilicon pattern 3a.

Note that phospho-silicated glass (PSG) layer, boron-silicated glass(BSG) or a stacked configuration thereof can be used instead of the BPSGof the insulating layer 6. However, silicon nitride is never used forthe insulating layer 6, since the silicon nitride prevents hydrogen frombeing diffused into the polycrystalline silicon pattern 3a.

Also, if the source region 31 and the drain region 32 are of alightly-doped drain (LDD) structure or the source region 31 and thedrain region 32 are shifted slightly from the gate electrode 5a, thegate silicon oxide layer 4 is removed from the source region 31 and thedrain region 32. Therefore, in this case, the non-doped silicon oxide ofthe insulating layer 6 is required to cover the source region 31 and thedrain region 32 again.

Next, referring to FIG. 4G, contact holes CONT1 and CONT2 are perforatedin the insulating layer 6 and the gate silicon oxide layer 4, so thatthe source region 31 and the drain region 32 are partly exposed.

Next, referring to FIG. 4H, an about 1 μm thick Al--Si--Cu alloy layer 7is deposited by sputtering. Then, the device is sintered at atemperature of about 400° C. for about 20 minutes in a H₂ /N₂ forminggas. As a result, an ohmic contact is realized between the regions 31and 32 and the Al--Si--Cu alloy layer 7.

Note that a barrier layer made of TiW, Ti, TiSi₂ or TiN can be providedbetween the regions 31 and 32 and the Al--Si--Cu alloy layer 7. Also,other metal such as AlSi can be used for the layer 7.

Next, referring to FIG. 4I, the Al--Si--Cu alloy layer 7 is patterned,so that electrode layers 7a and 7b are formed on the source region 31and the drain region 32, respectively.

Finally, referring to FIG. 4J, a hydrogen passivation is carried out forabout 30 minutes in a parallel-plate plasma reactor at a substratetemperature of about 350° C. with H₂ --Ar gas at a power density of 0.21W/cm² and a frequency of 30 kHz. Note that FIG. 5 is a plan view of thedevice of FIG. 4J, which is taken along the line IV--IV of FIG. 5.

In FIG. 4J, in the channel region 33, only dangling bonds of silicon atan interface with the gate silicon oxide layer 4 are combined withhydrogen. On the other hand, in the source region 31 and the drainregion 32, most of the dangling bonds of silicon therein are combinedwith hydrogen.

After that as occasion demands, a step for forming a passivation layeris carried out; however, in this case, such a step is carried out at atemperature lower than about 500° C. This will be explained later indetail.

In FIG. 6, which shows hydrogen passivation time in relation to drainvoltage characteristics where the gate length L is 6.0 μm and the gatewidth W is 2.0 μm, the hydrogen passivation time is changed in the firstembodiment. As explained above, a parasitic bipolar phenomenon may occurwhen the voltage V_(BODY) of the channel region 33 is 0.6V. As shown inFIG. 6, when the hydrogen passivation time is zero, the drain voltageV_(DS) is 6.5V at V_(BODY) =0.6V. Also, when the hydrogen passivationtime is 30, 60 and 150 minutes, the drain voltage V_(DS) is 6.4V, 6.0Vand 5.9V, respectively, at V_(BODY) =0.6V. That is, when the hydrogenpassivation time is 60 minutes, the reduction of the drain voltageV_(DS) at V_(BODY) =0.6V is saturated. Therefore, when the hydrogenpassivation time is larger than 60 minutes, a parasitic bipolarphenomenon may occur, so that the device cannot be applied to a 5Vsystem.

In FIG. 7, which shows hydrogen passivation time in relation tothreshold voltage characteristics where the gate length L is 6.0 μm, thegate width 2 is 2.0 μm, and the drain voltage V_(DS) is 5.0V, when thehydrogen passivation time is from 0 to 30 minutes, the threshold voltageV_(th) is remarkably reduced, and when the hydrogen passivation time isfrom 30 to 120 minutes, the threshold voltage V_(th) is graduallyreduced. That is, when the hydrogen passivation time is 120 minutes, thereduction of the threshold voltage V_(th) is saturated.

Also, as is not shown, when the hydrogen passivation time is 30 minutesunder the above-described condition, a leakage current is reducedseveral times (≈10⁻¹.5) (see 10⁻² in the prior art) as compared with acase where no hydrogen passivation is carried out. Also, an ON currentis increased some tens of hundreds of times (≈10⁴) (see 10⁵ in the priorart) as compared with a case where no hydrogen passivation is carriedout.

Thus, in the first embodiment, when the hydrogen passivation time is 30minutes, the breakdown voltage is remarkably improved, although theimprovement of the leakage current and the ON current are deterioratedslightly.

In FIG. 8, which shows hydrogen peak intensity characteristics forestimating the amount of hydrogen trapped in the polycrystalline siliconpattern 3a of FIGS. 4J and 5, devices as illustrated in FIGS. 9A and 9Bare heated by a thermal desorption spectroscopy (TDS) apparatus to about1150° C., and as a result, hydrogen degassed from the TDS apparatus isanalyd by a quadrupole mass spectrometer. Note that the device asillustrated in FIG. 9A is comprised of a fused quartz substrate 901, anon-doped silicon oxide layer 902, an about 80 μm thick P⁻ -typepolycrystalline silicon layer 903, an about 150 nm thick non-dopedsilicon oxide layer 904 formed by an LPCVD process, and an about 350 nmthick BPSG layer 905. On the other hand, the device as illustrated inFIG. 9B is the same as the device as illustrated in FIG. 9A excludingthe non-doped silicon oxide layer 904 and the BPSG layer 905.

First, after a plasma hydrogen passivation is performed upon the deviceas illustrated in FIG. 9A for 30, 60 and 150 minutes, the amount ofhydrogen degassed from the TDS apparatus is measured as indicated by asolid line in FIG. 8. In this case, the amount of hydrogen is remarkablyincreased for the hydrogen passivation time of 0 to 60 minutes, and theamount of hydrogen is gradually increased for the hydrogen passivationtime larger than 60 minutes.

Next, after a plasma hydrogen passivation is performed upon the deviceas illustrated in FIG. 9A for 30, 60 and 150 minutes, the BPSG layer 905is removed, and thereafter, the non-doped silicon oxide layer 904 isremoved by heavy hydrogen dilute fluorine acid to obtain the device ofFIG. 9B. At this time, hydrogen combined with dangling bonds of siliconat the interface with the non-doped silicon oxide layer 904 is replacedby heavy hydrogen. Then, the amount of hydrogen gassed from the TDSapparatus is measured as indicated by a dot line in FIG. 8. In thiscase, the amount of hydrogen trapped in the polycrystalline siliconlayer 903 is measured. That is, the amount of hydrogen trapped in thepolycrystalline silicon layer 903 is gradually increased for thehydrogen passivation time of 0 to 30 minutes and longer than 60 minutes.On the other hand, the amount of hydrogen trapped in the polycrystallinesilicon layer 903 is remarkably increased for the hydrogen passivationtime between 0 to 30 minutes.

The graph as shown in FIG. 8 reveals the following phenomenon. That is,dangling bonds of most silicon of the non-doped silicon oxide layer 904and dangling bonds of silicon of the polycrystalline silicon layer 903at the interface with the non-doped silicon oxide layer 904 are combinedwith hydrogen for the first 30 minutes of the hydrogen passivation.Thereafter, dangling bonds of silicon of the polycrystalline siliconlayer 903 are combined with hydrogen. In other words, hydrogen rapidlypropagates in the interface of the polycrystalline silicon layer 903with the non-doped silicon oxide layer 904, while hydrogen graduallypropagates in the interior of the polycrystalline silicon layer 903.

Also, generally, the hydrogen diffusion preventing power of BPSG is low,while the hydrogen diffusion preventing power of non-doped silicon oxideand polycrystalline silicon is high.

Returning to FIG. 4J, in view of the foregoing, hydrogen at theinterface of the channel region 33 with the gate silicon oxide layer 4is diffused from the interfaces of the source region 31 and the drainregion 32 with the gate silicon oxide layer 4.

Thus, in the first embodiment, the hydrogen passivation time ispreferably 30 minutes; however, it depends on the thickness of the gatesilicon oxide layer 4 and whether or not a BPSG layer is in directcontact with the regions 31 and 32.

Also, referring to FIGS. 6 and 8, when the hydrogen passivation time isvery long, for example, longer than 60 minutes, dangling bonds ofsilicon of the bulk of the channel region 33 of FIG. 4J are alsocombined with hydrogen. As a result, minority carriers (which are inthis case holes) generated in the channel region 33 are hardlyrecombined with electrons, so as to lengthen the life time of theminority carriers in the channel region 33, which may cause a parasiticbipolar phenomenon. As a result, the voltage V_(BODY) at the channelregion 33 easily rises.

As explained above, in the device of FIG. 9B, after the non-dopedsilicon oxide layer 904 (FIG. 9A) is removed by heavy hydrogen dilutefluoride acid, the hydrogen at the interface of the polycrystallinesilicon layer 903 and the non-doped silicon oxide layer 904 is replacedby heavy hydrogen. Therefore, when the device of FIG. 4B is heated bythe TDS apparatus, and heavy hydrogen degassed from the TDS apparatus isanalyzed by the quadrupole mass spectrometer, the amount of degassedheavy hydrogen is changed as shown in FIG. 10. That is, the amount ofdegassed heavy hydrogen is at peak when the temperature of the device ofFIG. 4B heated by the TDS apparatus is about 600° C. Also, most of theheavy hydrogen in the polycrystalline silicon layer 903 is degassed whenthe temperature of the device of FIG. 4B heated by the TDS apparatus isabout 700° C. Thus, as stated above, an operation for forming apassivation layer or the like on the device of FIG. 4J is carried outpreferably at a lower temperature than 500° C.

FIGS. 11A and 11B are cross-sectional views illustrating a secondembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention, and FIG. 12 is a plan view of the TFT of FIG.11B, which is a cross-sectional view taken along the line IX--IX of FIG.12. Note that FIGS. 11A, 11B and 12 correspond to FIGS. 4I, 4J and 5,respectively. That is, the manufacturing steps as illustrated in FIGS.4A through 4H are applied to the second embodiment.

Referring to FIG. 11A, an electrode layer 7a covers not only the sourceregion 31 entirely but also a part of the channel region 33 (also seeFIG. 12).

Finally, referring to FIG. 11B, a hydrogen passivation is carried in thesame way as in FIG. 4J. In this case, since the electrode layer 7a'covers the source region 31 and a part of the channel region 33,dangling bonds of silicon in the source region 31 are hardly combinedwith hydrogen.

As a result, as shown in FIG. 13, the second embodiment is advantageousover the first embodiment in respect to the breakdown voltage.

FIGS. 14A through 14E are cross-sectional views illustrating a thirdembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention, and FIG. 15 is a plan view of the TFT of FIG.14F, which is a cross-sectional view taken along the line IV--IV of FIG.15. Note that FIGS. 14A through 14E and 15 correspond to FIGS. 4Gthrough 4J and 5, respectively. Also, the manufacturing steps asillustrated in FIGS. 4A through 4F are applied to the third embodiment.

Referring to FIG. 14A, an about 100 nm thick polycrystalline siliconlayer is deposited on the insulating layer 6 by a CVD process, and ispatterned. As a result, a polycrystalline silicon pattern 8 is formed tocover the source region 31 and a part of the channel region 33.

Note that a silicon nitride layer can be used instead of thepolycrystalline silicon layer 8.

Next, referring to FIG. 14B, an about 300 nm thick BPSG layer 9 isdeposited by an APCVD process, and a heating operation is performed uponthe BPSG layer 9 to reflow it.

Further, in a similar way as that in FIG. 4G, contact holes CONT1 andCONT2 are perforated in the BPSG layer 9, the polycrystalline siliconlayer 8, and the insulating layer 6 and the gate silicon oxide layer 4,so that the source region 31 and the drain region 32 are partly exposed.

Next, referring to FIG. 14C, in the same way as in FIG. 4H, an about 1μm thick Al--Si--Cu alloy layer 7 is deposited by sputtering. Then, thedevice is sintered at a temperature of about 400° C. for about 20minutes in a H₂ /N₂ forming gas. As a result, an ohmic contact isrealized between the regions 31 and 32 and the Al--Si--Cu alloy layer 7.

Next, referring to FIG. 14D, in the same way as in FIG. 4I, theAl--Si--Cu alloy layer 7 is patterned, so that electrode layers 7a and7b are formed on the source region 31 and the drain region 32,respectively.

Finally, referring to FIG. 14E, in the same way as in FIG. 4J, ahydrogen passivation is carried out for about 30 minutes in aparallel-plate plasma reactor at a substrate temperature of about 350°C. with H₂ --Ar gas at a power density of 0.21 W/cm² and a frequency of30 kHz. In FIG. 4J, since the polycrystalline silicon pattern 8 coversthe source region 31 and a part of the channel region 33, dangling bondsof silicon in the source region 31 are hardly combined with hydrogen.

As a result, the third embodiment is advantageous over the firstembodiment in respect to the breakdown voltage.

FIGS. 16A through 16D are cross-sectional views illustrating a fourthembodiment of the method for manufacturing a top-gate type TFT accordingto the present invention. Also, the manufacturing steps as illustratedin FIGS. 4A through 4E are applied to the fourth embodiment.

Referring to FIG. 16A, an about 50 nm thick non-doped silicon oxidelayer 10 is deposited on the entire surface by an LPCVD process.

Then, an about 350 nm thick BPSG layer is deposited by an LPCVD processand a heating operation is performed thereupon to reflow it. Then, theBPSG layer is patterned, so that a BPSG pattern 11, which covers thedrain region 32 and does not cover the source region 31, is obtained.

Then, an about 500 nm thick non-doped silicon oxide layer 12 isdeposited by an LPCVD process. Further, a photoresist layer 13 is coatedon the silicon oxide layer 12.

Next, referring to FIG. 16B, the photoresist layer 13 and the siliconoxide layer 12 are etched back. As a result, a silicon oxide layer 12'is left, and the height of the silicon oxide layer 12' is approximatelythe same as that of the BPSG layer 11.

Next, referring to FIG. 16C, in a similar way as shown in FIG. 4G, acontact hole CONT1 is perforated in the silicon oxide layers 12' and 10and the gate silicon oxide layer 4, so that the source region 31 ispartly exposed. Simultaneously, a contact hole CONT2 is perforated inthe BPSG layer 11, the silicon oxide layer 10 and the gate silicon oxidelayer 4.

Next, referring to FIG. 16D, in the same way as in FIG. 4H, an about 1μm thick Al--Si--Cu alloy layer 7 is deposited by sputtering. Then, thedevice is sintered at a temperature of about 400° C. for about 20minutes in a H₂ /N₂ forming gas. As a result, an ohmic contact isrealized between the regions 31 and 32 and the Al--Si--Cu alloy layer 7.

Next, referring to FIG. 16E, in the same way as in FIG. 4I, theAl--Si--Cu alloy layer 7 is patterned, so that electrode layers 7a and7b are formed on the source region 31 and the frain region 32,respectively.

Finally, referring to FIG. 16F, in the same way as in FIG. 4J, hydrogenpassivation is carried out for about 30 minutes in a parallel-plateplasma reactor at a substrate temperature of about 350° C. with H₂ --Argas at a power density of 0.21 W/cm² and a frequency of 30 kHz. In FIG.16F. Since the source region 31 is covered by only non-doped siliconoxide, dangling bonds of silicon in the source region 31 are hardlycombined with hydrogen.

As a result, the fourth embodiment is advantageous over the firstembodiment in respect to the breakdown voltage.

Although the above-described embodiments relate to an N-channel typetop-gate type TFT, the present invention can be also applied to aP-channel type top-gate type TFT.

Also, in the above-described embodiments, an insulating substrate madeof non-doped monocrystalline silicon can be used instead of the fusedquartz substrate 1 and the silicon oxide layer 2.

As explained hereinbefore, according to the present invention, in achannel region of a top-gate type TFT, since only dangling bonds ofsilicon of a channel region at an interface with a gate insulating layerare combined with hydrogen, a bipolar parasitic phenomenon hardly occurstherein. As a result, the reduction of the breakdown voltage and thefluctuation of the threshold voltage can be suppressed.

What is claimed is:
 1. A method for manufacturing a top-gate type thinfilm transistor, comprising:forming a polycrystalline silicon layer on asubstrate; forming a gate insulating layer on said polycrystallinesilicon layer; forming a gate electrode on said gate insulatinglayer;introducing impurity ions into said polycrystalline silicon layerin self-alignment with said gate electrode to create source and drainregions in said polycrystalline silicon layer, a channel region of saidpolycrystalline silicon layer being sandwiched by said source and drainregions thereof; forming an insulating layer of said gate electrode andover said polycrystalline silicon layer; perforating first and secondcontact holes in said insulating layer and gate insulating layer, saidfirst and second contact holes reaching said source and drain regions,respectively; forming first and second metal electrodes in said firstand second contact holes, respectively, said first and second metalelectrodes being coupled to said source and drain regions, respectively;and performing hydrogen passivation upon said polycrystalline siliconlayer after said first and second metal electrodes are formed, so thatdangling bonds of silicon of said channel region at an interface withsaid gate insulating layer and dangling bonds of silicon of a part ofsaid source and drain regions are combined with hydrogen, wherein saidinsulating layer is made of a double configuration of non-doped siliconoxide and BPSG.
 2. A method as set forth in claim 1, wherein said firstmetal electrode covers only a part of said source region and said secondmetal electrode covers only a part of said drain region.
 3. A method asset forth in claim 1, wherein said hydrogen passivation step carries outsaid hydrogen passivation using hydrogenation by plasma discharge.
 4. Amethod as set forth in claim 3, wherein said hydrogen passivation stepcarries out said hydrogen passivation for a time period less thanapproximately 30 minutes.